(1) Field of the Invention
The invention relates to the removal of biofilms which form on outer surfaces of underwater optical devices. Despite the use of protective coatings on such optical devices to inhibit the growth of biofilms thereon, in due course biofilms adhere to the optical devices. The invention is directed to determining the adhesive strength of biofilms on protective coatings on such optical devices, and to determining what water pressures and water jet configurations and velocities are required to remove the biofilms and restore clarity to the optical device.
(2) Description of the Prior Art
Biofilm formation on underwater optical devices constitutes an initial step in the process of biofouling. Microorganisms, such as diatoms and bacteria, form colonies on surfaces in seawater. Once a biofilm is established, it serves as a foundation for barnacle larvae, ulna spores (“green sea lettuce”), and other macro-fouling organisms to settle, attach, and grow into macro-fouling colonies.
Long before macro-fouling occurs on optical devices, biofilm formation becomes problematic. Protective coatings are used on optical surfaces of underwater vehicles primarily for their water shedding capabilities upon surfacing. The coatings in current use are generally hydrophobic, meaning they cause water to shed off an optical device similar to rain drops sliding off leaves.
Unfortunately, even with the hydrophobic coatings on optical devices, biofilms tend to form and the coatings lose their hydrophobicity and the affected optical devices lose their optical clarity. In stationary seawater, biofilms can form within two weeks. Attempts to create coatings that are optically clear, hydrophobic and antifouling have thus far proven to be unsuccessful.
Typical methods for maintaining the optical integrity of underwater surfaces include surfacing of the underwater vehicle, wiping the optics with a suitable detergent, followed by a freshwater rinse. However, surfacing the underwater vehicle is not always an available option.
There is thus a need to clear off any accumulated biofilm without surfacing, and while remaining underwater. It has been proposed to use high pressure water jets, to remove biofouling, while the underwater vehicle is submerged and underway. An ideal resolution is to provide such a system as would be operative to prevent biofilms from affecting clarity of optical surfaces and disabling of water-shedding capabilities of the optical surfaces.
Efforts to provide a clear, hydrophobic coating for use in underwater optical systems are ongoing. An important input to such efforts is a thorough knowledge of the adhesive strengths of biofilms on various surfaces. As noted above, studies have centered on the use of high pressure water jets to remove macro-fouling and biofilms.
In order to test the strength of adhesion of biofilms to optical system surfaces and to test various configurations of jet nozzles and water pressures for application to the optical system surfaces, a test facility is required, inasmuch as extensive testing time on operational underwater vehicles is not an option.
There is thus a need for a test system which can be employed in a laboratory or other test facility, for an evaluation of the adhesion of biofilms and particular nozzle configurations and dimensions, and water ejection pressures and velocities, suitable for use on submarines.